It is a fact that tires constitute a major expense item in the budget of any road fleet owner. A tire casing can only be utilised to its full extent if it can be retreaded a number of times. Tire casings with damaged side walls cannot be retreaded with any degree of safety and must be discarded. Running a vehicle with an under pressure tire results not only in excessive wear of the tire tread but, of even more importance, in damage to the side walls which renders the tire casing unsuitable for retreading.
Under pressure tires are also a safety hazard in that it is not unknown for a soft tire to overheat to a temperature such that it bursts into flames. Furthermore, an under pressure tire is incapable of carrying its share of the axle load. Thus the other tires on the vehicle, and particularly the other tires on the same axle, are overloaded and their life is reduced. This problem is particularly acute when the underpressure tire is one of the tires of a pair of dual tires. In such circumstances, the other tire of the pair carries the entire load with extremely detrimental effects on its life.
For all these reasons it has long been accepted that vehicles, particularly heavy duty vehicles, should be fitted with means for warning the driver that he has an under pressure tire. A multitude of different proposals have been made in the past to provide such warning means but, to the best of applicant's knowledge, very few of these became available on a commercial scale, and those which did appear to have been a failure. At this time no warning systems seem to be available commercially in the United States.
The reasons why the systems that have been proposed have failed are numerous. Many proposed devices are over complex and/or too delicate to operate for any length of time when subjected to the constant drumming and vibration of a vehicle wheel. In other cases, to fit the warning structure, the wheel had to be modified, for example, by being drilled or by having some components welded or otherwise secured thereto, and this appears to be generally unacceptable to the truck user. It must also be remembered that, if a wheel has to be changed, the unit must be capable of being removed and refitted by the driver at the roadside.
A major problem which has received attention in the past is that of transferring a signal indicating a low pressure condition from the rapidly rotating wheel to the driver. Sounds such as whistles, explosions and the noise of an element hitting the pavement, have all been proposed. Radio transmitters and receivers and sound emitters and transducers have also been proposed as possible means for transferring the signal from the wheel to the vehicle body for onward transmission to the driver.
An object of the present invention is to provide a low pressure warning system which is simple in construction and capable of withstanding the vibrations and shocks to which it is subjected in use.
Another object of the present invention is to provide a simple, effective and rugged means for transferring a signal indicative of low pressure from the wheel to the vehicle body for onward transmission to the driver.
Another object of the present invention is to provide a system for warning that a tire is under inflated, which device can be removed from the wheel and refitted without the use of tools.
A difficulty which is encountered with dual tires is that the air inflation valve of the inner tire is difficult to reach with the air line. A further object of the present invention is to provide a device for warning that a tire is at below pressure and which additionally permits the tires of a pair of dual tires to be inflated from a single inlet.
Circumstances can arise where one tire of a pair is leaking slowly and eventually reaches a pressure at which the warning device is activated. A still further object of the present invention is to provide a low pressure warning device in which, by a simple action, the pressure in the tires of a pair of dual tires can be equalised.